Here's how my Coupe is set up:

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That pillar mount looks like where a ring for shoulder belts fit With the belt running Up from the floor/side of rocker from either an anchor or retractor.
The rear mount points are threaded holes in the parcel shelf. It's easier to loacte them if you either get in the trunk and look up at the parcel tray on the outside ends.
The lower points can be accessed by removing the lower seat squab .If they have never been used there will be plastic bungs in the holes or masking tape over them.
The front belt mounts can be found either getting under the car and looking up at the tunnel around the mid point or removing the carpet .the side mounts are generally in the center point of the B pillar section about 25 mm above the floor.
Otghers have pointed out how to find the upper mounts,either 1/2 way up the B pillar or on top of the side panel inside of the window.
You can mount retractors up on the parcel shelf but the word " Ugly " comes to mind

Retractors Are not safer than regular lap and diagonals,Just more convenient.

What must be remembered is that the belt MUST NOT PULL DOWN ,thats why the belt mount is up high on the side . If it runs over the rear of the seat so it pulls you down in an accident it will destroy your spine

That's why I don't care for the shoulder belts in the older coupes. They are too low and tend to pull down. Newer coupes attach their shoulder straps to the roof
But, as I mentioned, a '61 coupe probably has no factory belt anchors anyway.
When I installed seatbelts in my '60 Fintail, I went with non-retracting lap & shoulder belts made by Beams Manufacturing of Oklahoma City, and sold through J C Whitney. The belts were availible in several colors and had the old '60s-style chrome buckles, but weren't availible with retractors. Using the seatbelt mounting locations in my '72 250 sedan as a pattern, I installed the anchors for my Fintail's lower belts to the inner-sill and tunnel. I had to fabricate a pair of reinforced anchor plates, then cut axcess-holes in the pillars to install them. I went with new belts because late-model used belts couldn't be easily adapted to my Fintail, and I didn't want to trust vintage seatbelts in a car I planned to be driving regularly.
But as the original post said, If it's mainly for show, lap belts may be sufficient.

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DrDKW

I have a W114 1972 Mercedes 250 with 3 point belts but they are hook and flip old school style. I would really love to replace these with push button 3 point belts. Does anyone have a recommendation on how to go about doing so.

I have some on ebay that say with some adjusting they can be used for my car model. I am skeptical though.

The ******** I spoke to wanted to charge $200/hour for labor and $250 for the belts. That seem outrageous.

Can anyone point me in the right direction?

Go to your nearest mercedes dealer and order a set of new ones.Fit them yourself in a few minutes each side. There are only three bolts involved per side.